A three dimensionally bent tube consisting a conventional protective tube of this kind for buried cables generally consists of a combination of hard two-dimensionally bent tubes having a predetermined radius of curvature (for example, 3 mR, 5 mR and 10 mR), or a hard corrugated polyethylene tube, or a mechanically bent metal tube. However, when such tubes are employed, it is necessary to select bent tubes of a plurality of radii of curvature and cover cables therewith by giving consideration to the directions in which these tubes are bent. Consequently, much labor and technical skill are required. When hard corrugated polyethylene tubes are used, a worker damages a cable in some cases by accidentally piercing his pickax through a thin-walled portion thereof during a cable burying operation.
When metal tubes are used, it is necessary to carry a bending machine into a construction field and repeatedly field-adjust the radii of curvature and bending directions of the metal tubes, so that complicated operations are necessarily carried out in a cable burying place. Consequently, much labor is required, and a construction period is prolonged. Moreover, once in a long while, pipes laid underground are damaged in error by working tools during the engineering work for water supply pipes, sewerage pipes and gas pipes.